This invention relates to a coating apparatus. Specifically, the present invention is an improved extruder for extruding a coating onto a support.
The term "support" as used herein is intended to mean a flexible belt-shaped material 0.3 to 3 m in width, 45 to 10,000 m in length and 5 to 200 mm in thickness, which is made of either a plastic film of polyethylene terephthalate, polyethylene-2, 6-naphthalate, cellulose diacetate, cellulose triacetate, cellulose acetate prpionate, polyvinyl chloride, polyvinylidene, polycarbonate, polyimide or polyamide; paper; paper on which a-polyolefin having two to ten carbons such as polyethlene, polypropyrene or ethylene butane copolymer is coated or laminated; or metal foil or aluminum, copper or tin. The support includes a belt-shaped material which is obtained by forming a preliminary treatment layer on the aforementioned belt-shaped material.
Samples of coating solutions include a photographing photosensitive solution, a magnetic solution, a surface protecting solution, a charging preventing solution or a smoothing solution. The particular solution extruded onto the surface of the support depends upon the use intended. After the coating solution dries, the coated support is cut into pieces having a desired width and length. Typical products of the support are photographing films, print paper and magnetic tapes.
Japanese Patent Application Laid-Open No's 138036/1975 and 90350/1978 disclose extruders with doctor edges which can apply an extremely small and uniform thickness coating solution to a support which is run at a high speed, the speed being between 50 and 100 m/min.
A common feature of these extruders is that the support is laid in such a manner as to be able to bend in the direction of its thickness between support guiding means such as guide rollers. Another common feature is that protruded surfaces of a doctor edge and a back edge, which form a part of the front end portion of the extruder where a slot outlet for extruding the coating solution is formed, confront the support in such a manner so as to abut against the surface of the support. In addition, the distance between the support and the doctor edge provided on the downstream side of the support is changed according to the quantity of supply of extrusion of the coating solution extruded through the slot when carrying out the coating operation.
In the former Japanese Patent Application Laid-Out No. 13803/1975, the doctor edge has a long edge length, and the edge surface is a curved surface having a large curvature, a polygonal surface, an uneven surface, or a completely flat surface. The doctor edge thus formed is confronted with the surface of the support. Therefore, if the section of the doctor edge is not uniformly machined and finished also in the widthwise direction of the support, then the thickness of the coated film is unavoidably considerably non-uniform in the widthwise direction. Accordingly, in machining the doctor edge, high machining precision is required, and a material for manufacturing the doctor edge is limited.
Recently, there has been a strong demand for a coated film more uniform in thickness. However, in the case of the doctor edge having the above-described edge surface, the ends of the doctor edge or the parts in the vicinity thereof which greatly affect the surface quality, thickness, etc. of the coated film are not precisely straight because the ends of the doctor edge are rounded or chamfered to remove burrs. Therefore, the extruder is limited in controlling the thickness of the coated film in the order of micrometers (.mu.m).
Even if it were possible to machine and finish the ends of the doctor edge in the order of micrometers, the surface and ends of the doctor edge would be readily worn, because when coating, the doctor edge is often kept in contact for a long time with a coating solution containing a relatively large quantity of relatively hard and minute foreign particles such as ferromagnetic iron oxide powders, silver halogenide particles and abrasive particles. Therefore, it is necessary to correct the portions of the doctor edge worn by abrasion. The accuracy of the coated finish is greatly lowered whenever abrasion occurs.
In the latter Japanese Patent Application Laid-Out No. 90350/1978, the doctor edge also has a flat surface and rounded or chamfered ends. In addition, a protruded surface of the back edge located on the upstream side of the support with respect to the slot outlet contacts the surface of the support. Therefore, as in the former extruder referred to above, it is difficult to provide a coated film uniform in thickness and satisfactory in surface quality.
Furthermore, both of these prior art extruders are disadvantageous in that, as the quantity of the above-described abrasive particles is increased, defects such as longitudinal stripes are liable to be formed which degrade the product quality.
In the case where the doctor edge has a flat surface, the coated surface can be readily improved in accuracy. However, since the doctor edge has an upstream edge and a downstream edge, one of these two edges pushes the support surface more forcefully than the other edge resulting in the doctoring operation on the flat surface being non-uniform. As a result, the tension of the support is abruptly varied (increased) at the end pushing more forcefully and accordingly it is difficult to form a coated film having a desired thickness, and the aforementioned longitudinal stripe defects are frequently formed.